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Calcium: Friend or Foe?

Calcium is both a mineral and a nutrient. It is important for healthy bone. Calcium is stored mostly in the skeleton. However, calcium is important for many body functions. Calcium is found in muscle, the lining of blood vessels, and body fluids. Calcium is important for bone strength, muscle action, and to allow blood vessels to contract and relax. More than 99% of the body’s calcium is found in bone, where it is stored until the body needs it.

SOURCES OF CALCIUM
Certain foods have calcium (see table 1) as do calcium supplements. Dairy products such as milk, yogurt, and cheese are rich sources of calcium and provide most of the dietary calcium consumed in the United States. Vegetables
have small amounts of calcium, and all other foods have very little calcium, except fortified juices and cereals.

About 43% of adults and 70% of older women in the US take calcium. There is no one ‘best’ calcium supplement. The two most commonly used calcium supplements are calcium carbonate and calcium citrate. Calcium carbonate
has more active calcium; 40% compared to 21% for calcium citrate. Thus, you can take fewer calcium pills if you take calcium carbonate. This may make it easier for you to take your pills and may also make it less expensive. Calcium carbonate comes in a more readily available chewable form; however, calcium citrate chews are also available. Constipation and bloating are more common with calcium carbonate than with calcium citrate. People with a history of calcium-containing kidney stones should ask their doctor how to take calcium safely.

GUT ABSORPTION OF CALCIUM
Not all of the calcium you take is available to the body to use. Things that affect how the intestines absorb calcium include the amount of calcium taken at any one time, the amount of vitamin D in the body, the health of the cells in the gut, and aging. Unlike calcium citrate, calcium carbonate needs stomach acid or acid food to help dissolve the tablet in the stomach so that the gut can absorb it. So to get the most benefit from the calcium carbonate pill you are taking, take it with your meals!

Table 1: Calcium content of common foods, in milligrams (mg) of elemental calcium

Vitamin D is needed for the body to absorb calcium. On average, you absorb about 25% of the calcium you consume (but only 10% when vitamin D levels in the body are low, and up to 30% when vitamin D levels are normal). Calcium is better absorbed at higher doses, but about the most that the body can absorb at any one time is about 500-600 milligrams of elemental calcium. Taking more than that in one dose may lead to kidney stones. So, try to get most of your calcium through food sources. If you are using calcium supplements, divide your intake into two or more doses during the day with meals (for calcium carbonate or citrate) or between meals (for calcium citrate only).

DOSING OF CALCIUM INTAKE
The recommended daily allowance (RDA) of a nutrient is how much is estimated to maintain a healthy diet. In 2011 the Institute of Medicine (IOM) published an RDA for calcium and vitamin D for all age groups (see table 2). The RDA for calcium goes up during certain periods in life—during the teenage years, during pregnancy and lactation, and as we grow older.

Patients with bone loss (osteopenia [os-tee-oh-PEEN-neeuh] or osteoporosis [os-tee-oh-puh-ROH-sis]) and people with diseases that affect the gut may need higher doses of calcium and/or vitamin D. Calcium should be used cautiously in people who have conditions that cause too much calcium in the blood, such as parathyroid [para-THIGH-roid] gland disorders. In general, calcium intake should follow IOM guidelines (table). Talk to your doctor if you think you should take more or less than stated in IOM guidelines.

BENEFIT AND SAFETY OF CALCIUM INTAKE
The body needs calcium for skeletal growth and repair. Bone is an active tissue, and calcium is lost from the skeleton when bone formation cannot keep up with bone loss. This is especially true after menopause in women, and in older
men. Adequate calcium intake may help reduce fractures by 10%. The benefit of calcium and vitamin D on bone health is much higher in persons with low vitamin D levels. There may be a protective effect of calcium and vitamin D against
breast and prostate cancer, and the supplements may help reduce the risk for colon cancer. However, there isn’t enough evidence to show a true benefit of calcium and vitamin D on reducing or raising the risk of any cancer.

Although recent summaries of calcium and vitamin D studies suggest a possible link between calcium and vitamin D supplements and cardiovascular (CVD), the IOM review could not find evidence that calcium and vitamin D caused any CVD events. Until we learn more, take enough calcium to meet RDA requirements while avoiding taking too much. Be sure that the total amount of calcium from food and supplements does not exceed the RDA requirements.

Dr. Daniel L. Hurley is a consultant in the Department of Medicine and Division of Endocrinology, Diabetes, Metabolism, and Nutrition at Mayo Clinic. He received his Internal Medicine and Endocrinology training at Mayo Graduate School of Medicine. Dr. Hurley was awarded the Randall G. Sprague Award for Outstanding Achievement as an Endocrine fellow, the Department of Medicine Teacher of the Year Award, and the Henry S. Plummer Distinguished Physician Award for the Department of Medicine. His clinical interests include metabolic bone disease, nutritional health, and mentoring endocrine fellows and staff physicians new to Mayo Foundation." style="float:left; padding:5px;">

Table 2:Institutes of Medicine RDA for Calcium and Vitamin D

a RDA = i ntake that covers needs of 97.5% of the healthy normal population.
b Reflects Adequate Intake (AI ) reference value rather than RDA. RDAs have not been established for infants due to insufficient data.
c Calcium and vitamin D RDAs are the same for pregnant or lactating females in these age groups.

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